Biology Reference
In-Depth Information
BAK1 may be a signal “amplifi er” rather than an integral component of down-
stream signaling pathways (Nicaise et al.
2009
). BRI1-BAK1 interaction leads to
the transphorylation of their respective kinase domains and the subsequent
enhancement of BRI1 signaling output (Wang et al.
2008a
), suggesting that BAK
is a signal amplifi er rather than an integral component of downstream signaling
pathways (Nicaise et al.
2009
). The PRRs for both fl g22 and AtPep1 associate
with the interacting receptor partner, BAK1, and likely activate cyclic nucleo-
tide-gated calcium channels via receptor guanylyl cyclase activity (Ma et al.
2009
; Postel et al.
2010
).
2.12.6
BIK1
Downstream of PAMP-PRR-BAK1 signaling complex, several receptor-like
cytoplasmic kinases (RLCKs) play important role in regulation of the signaling
pathways. Lacking an apparent extracellular domain, RLCKs more likely func-
tion in signal transduction rather than in signal perception (Lu et al.
2010
). An
RLCK member BIK1 (Botrytis-induced kinase 1) plays an important role in
mediating early fl agellin signaling from the FLS2/BAK1 receptor complex (Lu
et al.
2010
). Flg22-induced oxidative burst has been shown to be reduced in
bik1
mutant
Arabidopsis
plants, suggesting the importance of BIK1 in plant innate
immune system (Zhang et al.
2010a
). BIK1 was originally identifi ed as a com-
ponent in plant defense against necrotrophic fungal pathogens (Veronese et al.
2006
).
Both FLS2 and BAK1 are able to interact with and phosphorylate the receptor-
like cytoplasmic kinase BIK1, which seems to act as positive regulator of the FLS2
signaling pathway (Lu et al.
2010
; Zhang et al.
2010a
). BIK1 forms a complex with
unstimulated FLS2 in plants, and fl g22 induces a rapid phosphorylation of BIK1 in
both an FLS2- and BAK-dependent manner (Zhang et al.
2010a
). BIK1 is phos-
phorylated within 1 min upon fl agellin perception (Wu et al.
2011
). BIK1 is rapidly
phosphorylated by fl g22 within the fi rst minutes after stimulation, which may hap-
pen instantaneously with the formation of FLS2/BAK1 complex (Lu et al.
2010
).
In vivo
and
in vitro
data suggest that BIK1 associates with both FLS2 and BAK1.
BIK1 is a substrate of BAK1, whereas BAK1 and FLS2 are also substrates of BIK1,
suggesting transphosphorylation events between BIK1 and the FLS2/BAK1
complex. BIK1 is phosphorylated by BAK1, and BIK1 also directly phosphorylates
BAK1 and FLS2
in vitro
. The fl agellin phosphorylation site Thr
287
of BIK1 is
required for its phosphorylation on BAK1 and FLS2, suggesting that BIK1 is likely
fi rst phosphorylated upon fl agellin perception and subsequently BIK1 transphos-
phorylates FLS2/BAK1 to propagate fl agellin signaling (Lu et al.
2010
). BIK1
appears to function downstream of FLS2/BAK1 complex formation and phosphory-
lation because BIK1 phosphorylation requires not only the presence of both FLS2
and BAK1, but also their kinase activity (Lu et al.
2010
).
Search WWH ::
Custom Search